There is a strong demand for systems or sensors that can detect the presence of hazardous materials, such as explosive materials, and in particular systems with high sensitivity and specificity, as well as the potential for standoff detection. Primary, secondary and tertiary explosives make up the three classes of high explosive materials, each having decreasing sensitivity to shock, friction, and heat. Peroxide-based explosives (e.g., acetone peroxides) are one of the main constituents of primary explosives, while nitro-based explosives make up the majority of secondary explosives (e.g., trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), pentrite (PETN)), and tertiary explosives (e.g., ammonium nitrate/fuel oil (ANFO)).
Raman spectroscopic techniques have been shown to provide high specificity in the identification of compounds. However, detection of selective high explosive materials using Raman-based sensors has limited sensitivity due to the weak Raman scattering, particularly when explosive materials are present in low concentrations, such as in the vapor phase (exemplified by high-vapor pressure peroxide-based species). On the other hand, fluorescence detection techniques are highly sensitive, typically several orders of magnitude more sensitive than Raman techniques, by comparison.
Direct detection of explosives using native fluorescence of the target substance is challenging because the fluorescence spectra are typically broad and structureless/featureless. Selective photofragments from photodissociation of explosive materials have strong fluorescence that produces structured or feature-evident spectra. Nitric oxide (NO) is a characteristic photofragment of nitro-based explosive materials when irradiated with ultraviolet (UV) light. Specifically, absorption by NO via its various A-X (v′,v″) bands, e.g. (0,0), (1,1), (2,2), and (0,2) transitions near 226, 224, 222, and 248 nm, results in discrete laser-induced fluorescence (LIF) emissions.
In the case of peroxide-based materials, hydroxyl radical (OH) may be the ultimate photofragment. Similarly, absorption by OH via its various A-X (v′,v″) bands, e.g. (1,0), (0,0), (1,1), (2,0) transitions near 282, 309, 315, 262 nm, results in discrete LIF emissions. The unique fluorescence spectral fingerprint of NO or OH can serve as a high-confidence indicator for nitro-based or peroxide-based materials, respectively, with detection sensitivities higher than the Raman signatures of their respective parent target molecules. The discrete structures in the molecular fingerprints of NO and OH are characteristic of diatomic molecules and yield distinctive fluorescence spectra in contrast to broad fluorescence profiles of larger molecules that have multiple pathways of energy disposal for populations at the excited energy levels.
There is an opportunity to exploit the unique fluorescence spectra of certain daughter photofragment molecules of a target material in order to detect the presence of the target material based on captured Raman spectra and fluorescence spectra.